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The authors present a case of unilateral agenesis of internal carotid artery associated with an aneurysm at the unusual site. A 22-years old female was admitted with sudden headache and nausea. She was alert on admission and no focal neurologic deficit was detected. Brain computed tomography(CT) scan showed subarachnoid hemorrhage. In the cerebral angiography, right internal carotid artery was absent and an aneurysm was arisen from the junction of the right middle cerebral artery and posterior communicating artery. The blood supply of the right anterior cerebral artery was maintained from the left internal carotid artery via the anterior commuicating artery. The territory of the right middle cerebral artery was supplied from the basilar artery through enlarged right posterior communicating artery. Direct clipping of the aneurismal neck was performed through right pterional approach. In the postoperative high resolution CT scan, there was no carotid canal in the right skull base suggesting agenesis of the right internal carotid artery. Key words:Internal carotid artery;Agenesis;Aneurysm;Collaterral circulation.